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Register a new userThe link between the assembly of the inner dark matter halo and the angular momentum evolution of galaxies in the EAGLE simulation
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Jesus Zavala Franco
Publication date
2015
fields
Physics
and research's language is
English
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We explore the co-evolution of the specific angular momentum of dark matter haloes and the cold baryons that comprise the galaxies within. We study over two thousand central galaxies within the reference cosmological hydrodynamical simulation of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. We employ a methodology within which the evolutionary history of a system is specified by the time-evolving properties of the Lagrangian particles that define it at z=0. We find a strong correlation between the evolution of the specific angular momentum of todays stars (cold gas) and that of the inner (whole) dark matter halo they are associated with. This link is particularly strong for the stars formed before the epoch of maximum expansion and subsequent collapse of the central dark matter halo (turnaround). Spheroids are typically assembled primarily from stars formed prior to turnaround, and are therefore destined to suffer a net loss of angular momentum associated with the strong merging activity during the assembly of the inner dark matter halo. Stellar discs retain their specific angular momentum since they are comprised of stars formed mainly after turnaround, from gas that mostly preserves the high specific angular momentum it acquired by tidal torques during the linear growth of the halo. Since the specific angular momentum loss of the stars is tied to the galaxys morphology today, it may be possible to use our results to predict, statistically, the assembly history of a halo given the morphology of the galaxy it hosts.
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Despite the insights gained in the last few years, our knowledge about the formation and evolution scenario for the spheroid-dominated galaxies is still incomplete. New and more powerful cosmological simulations have been developed that together with more precise observations open the possibility of more detailed study of the formation of early-type galaxies (ETGs). The aim of this work is to analyse the assembly histories of ETGs in a $Lambda$-CDM cosmology, focussing on the archeological approach given by the mass-growth histories.We inspected a sample of dispersion-dominated galaxies selected from the largest volume simulation of the EAGLE project. This simulation includes a variety of physical processes such as radiative cooling, star formation (SF), metal enrichment, and stellar and active galactic nucleus (AGN) feedback. The selected sample comprised 508 spheroid-dominated galaxies classified according to their dynamical properties. Their surface brightness profile, the fundamental relations, kinematic properties, and stellar-mass growth histories are estimated and analysed. The findings are confronted with recent observations.The simulated ETGs are found to globally reproduce the fundamental relations of ellipticals. All of them have an inner disc component where residual younger stellar populations (SPs) are detected. A fraction of this inner-disc correlates with bulge-to-total ratio. We find a relation between kinematics and shape that implies that dispersion-dominated galaxies with low $V/sigma_L$ (where $V$ is the average rotational velocity and $sigma_L$ the one dimensional velocity dispersion) tend to have ellipticity smaller than $sim 0.5$ and are dominated by old stars. Abridged
We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the EAGLE cosmological hydrodynamical simulation. We find, in striking concurrence with recent observational studies, that the <SFR>--BHAR relation for an AGN selected sample produces a relatively flat trend, whilst the <BHAR>--SFR relation for a SFR selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time-averaged over a period of 100~Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M200 $lesssim 10^{11.5}$Msol grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range $10^{11.5} lesssim$ M200 $lesssim 10^{12.5 }$Msol BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M200 $gtrsim 10^{12.5}$Msol) both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR $sim 10^{3}$. Given the complexity of the full SFR--BHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the <SFR>--BHAR and <BHAR>--SFR relationships are due to sampling considerably different regions of this plane.
We investigate the connection between the morphology and internal kinematics of the stellar component of central galaxies with mass $M_star > {10}^{9.5} {rm M}_odot$ in the EAGLE simulations. We compare several kinematic diagnostics commonly used to describe simulated galaxies, and find good consistency between them. We model the structure of galaxies as ellipsoids and quantify their morphology via the ratios of their principal axes. We show that the differentiation of blue star-forming and red quiescent galaxies using morphological diagnostics can be achieved with similar efficacy to the use of kinematical diagnostics, but only if one is able to measure both the flattening and the triaxiality of the galaxy. Flattened oblate galaxies exhibit greater rotational support than their spheroidal counterparts, but there is significant scatter in the relationship between morphological and kinematical diagnostics, such that kinematically-similar galaxies can exhibit a broad range of morphologies. The scatter in the relationship between the flattening and the ratio of the rotation and dispersion velocities ($v/sigma$) correlates strongly with the anisotropy of the stellar velocity dispersion: at fixed $v/sigma$, flatter galaxies exhibit greater dispersion in the plane defined by the intermediate and major axes than along the minor axis, indicating that the morphology of simulated galaxies is influenced significantly by the structure of their velocity dispersion. The simulations reveal that this anisotropy correlates with the intrinsic morphology of the galaxys inner dark matter halo, i.e. the halos morphology that emerges in the absence of dissipative baryonic physics. This implies the existence of a causal relationship between the morphologies of galaxies and that of their host dark matter haloes.
We introduce the Virgo Consortiums EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to $lesssim 0.2$ dex over the full mass range, $10^8 < M_*/M_odot lesssim 10^{11}$, a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully-Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic CIV and OVI. While the mass-metallicity relations for gas and stars are consistent with observations for $M_* gtrsim 10^9 M_odot$, they are insufficiently steep at lower masses. The gas fractions and temperatures are too high for clusters of galaxies, but for groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. EAGLE constitutes a valuable new resource for studies of galaxy formation.
We present the evolution of galaxy sizes, from redshift 2 to 0, for actively star forming and passive galaxies in the cosmological hydrodynamical 1003 cMpc3 simulation of the EAGLE project. We find that the sizes increase with stellar mass , but that the relation weakens with increasing redshift. Separating galaxies by their star formation activity, we find that passive galaxies are typically smaller than active galaxies at fixed stellar mass. These trends are consistent with those found in observations and the level of agreement between the predicted and observed size - mass relation is of order 0.1 dex for z < 1 and 0.2-0.3 dex from redshift 1 to 2. We use the simulation to compare the evolution of individual galaxies to that of the population as a whole. While the evolution of the size-stellar mass relation for active galaxies provides a good proxy for the evolution of individual galaxies, the evolution of individual passive galaxies is not well represented by the observed size - mass relation due to the evolving number density of passive galaxies. Observations of z approx 2 galaxies have revealed an abundance of massive red compact galaxies, that depletes below z approx 1. We find that a similar population forms naturally in the simulation. Comparing these galaxies to their z = 0 descendants, we find that all compact galaxies grow in size due to the high-redshift stars migrating outwards. Approximately 60% of the compact galaxies increase in size further due to renewed star formation and/or mergers.
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